A hypothesis is presented to explain cell recognition, selective cell adhesion, cell sorting out in embryonic cells, the invasiveness of normal tissue by tumor cells and the general problem of metastasis of cancer cells. The hypothesis states that cells sort out by non-random movement leading to a segregation of cells according to species and/or cell type. The sorting out is accompanied by the formation of microvilli in specific response to neighboring cells and the production of a specific extracellular material to bind similar cells together. The following studies using sea urchin embryonic cells, are proposed to test the hypothesis: to study the role of microtubules and microfilaments in the reaggregation process, to study microvillus formation, to further characterize the extra-cellular material and to determine its role in cell adhesion, to determine the role of the paternal genome in the reaggregation of embryonic cells, to study the effects of animalizing and vegetalizing agents on cell types and their relationship to normal morphogenetic movements and to study the formation of primary mesenchyme from micromeres. We will also study reaggregation and sorting out in the sand dollar embryo.